Question

part a
which of the following statements about physical and chemical changes is false?
chemical reactions are chemical changes.
phase changes are always physical changes.
in a chemical change, matter changes its composition.
in a physical change, matter does not change its composition.
all of the above statements are true.

Explanation:

• Chemical reactions: By definition, chemical reactions involve chemical changes where new substances are formed. So, "Chemical reactions are chemical changes" is true.
• Phase changes: Phase changes (e.g., melting of ice to water, boiling of water to steam) only change the state of matter (solid - liquid - gas) without altering the chemical composition. So, "Phase changes are always physical changes" is true.
• Chemical change: In a chemical change, bonds are broken and new bonds are formed, which means the composition of matter changes. For example, when hydrogen and oxygen react to form water (\(2H_2+O_2 = 2H_2O\)), the composition changes. So, "In a chemical change, matter changes its composition" is true.
• Physical change: In a physical change, such as cutting a piece of paper or dissolving salt in water (where salt can be recovered by evaporation), the chemical composition of the substances involved remains the same. So, "In a physical change, matter does not change its composition" is true.

Since all the first four statements are true, the statement "All of the above statements are true" is also valid.

Answer:

All of the above statements are true.